The Atkins craze is pretty much behind us -- my neighborhood low-carb food store recently shuttered its doors -- so now it's time to circle back with some good news about carbohydrates.
High-carb foods such as beans, bananas and whole-grain breads might have gotten the cold shoulder in recent years. But of late, some in the food industry have begun touting the health benefits of "resistant starch" -- a form of fiber that delivers some of the health benefits of soluble and insoluble fibers.
Starches are simply long chains of sugar molecules that must be broken apart by amylase, an enzyme in the saliva and small intestine, releasing the individual sugars so that they can be absorbed.
But resistant starches defy digestion: They contain more of a tight-packing form of starch that amylase doesn't easily attack. They continue unscathed through the digestive tract to the large intestine, where bacteria feed on them, fermenting them and producing fatty acids.
These fatty acids (acetate, propionate and butyrate) contribute to colon health. They make the environment more acidic and less hospitable to bacteria that can cause illness. Absorption of potentially toxic or carcinogenic compounds is lowered too.
Like insoluble fibers such as cellulose, resistant starch traps water and adds bulk to the stool, which helps with regularity. And it appears to share properties with soluble fibers that help to control blood sugar and cholesterol.
In a Korean study published in 2004 in the Journal of Nutritional Science and Vitaminology, overweight subjects who ate resistant starch every day for three weeks had significant drops (about 50 points, on average) in their cholesterol levels. The placebo group, who ate regular starch, showed no such change. But the subjects in this study ate a lot of resistant starch -- nearly five times the estimated 5 grams Americans eat daily via whole foods.
More modest intakes of resistant starch have been shown in a few studies to help blunt rapid rises in blood sugar after a carbohydrate-rich meal. In a Japanese report, for example, blood sugar levels in subjects with borderline diabetes rose 20% less after they ate bread laced with 6 grams of resistant starch than after a meal of bread without resistant starch.
Foods high in resistant starch may help with weight loss too. In a 2006 study from Louisiana State University published in the journal Obesity, rats fed chow supplemented with resistant starch lost body fat -- but not because the fiber filled them up. Instead, resistant starch stimulated genes that contain the blueprint for two hormones, PYY and GLP-1, that make us feel full and send signals to stop eating.
A small study at the University of Colorado, published in 2004 in Nutrition & Metabolism, looked at the effects of resistant starch on fat metabolism in people after eating a single meal. The fat in the meal was chemically labeled so that the end product of its metabolism -- carbon dioxide released in the breath -- could be traced over a six-hour period.
Fat burning was 23% higher in those who ate 5 grams of resistant starch in the meal than in those who didn't: More fat was burned for fuel and less was available to store away. The subjects in this study were of normal weight and they ate the starch once. Longer studies of intake in overweight people would be needed to see if weight loss would occur.
Resistant starches exist naturally in underripe bananas, navy beans, lentils, barley and whole-grain breads -- but in relatively small amounts. One review of the literature estimated that 6 to 12 grams of resistant starch at a meal can exert beneficial effects on blood sugar, but that daily intakes in the range of 20 grams might be needed to provide the bulk to promote digestive health. Even if you like your bananas green, a single fruit -- one of the richest natural sources -- will provide just less than 5 grams.
Food manufacturers are on top of this story, though. Forget low-carb breads and candy bars, so popular a few years ago. Corn-derived resistant starch is making its way into breads, cereals and pastas -- coming to a store near you.
Susan Bowerman is a registered dietitian and assistant director of the UCLA Center for Human Nutrition.